1. Field of the Invention
The present invention relates to a method of forming membranes in general, and, more specifically, to a method of forming silicon membranes in which a silicon device is to be formed.
2. Description of the Related Art
In the past, thin silicon membranes suspended by a substrate, such as a silicon substrate, have been used in several fields. These fields include x-ray lithography in which a thin suspended silicon membrane, which is transparent to X rays, is coated with a metal such as gold. The coated membrane is then used as a mask for X rays.
Thin silicon membranes have also been used for specialty electronic devices such as air bridges in fast transistors. Another example of such specialty electronic devices is a balometer, which detects single photons to measure heat, for example an x-ray balometer.
Among the prior methods of forming silicon membranes is a method in which a silicon boron layer is formed on a silicon substrate. A silicon cap layer is then formed on the silicon boron layer. A portion of the silicon substrate is then removed by etching, with the silicon boron layer serving as an etch stop. Finally, the silicon boron layer may or may not be removed to form a silicon membrane.
There are several problems associated with using silicon boron as the etch stop layer in forming silicon membranes. For example, it is sometimes necessary that the silicon membrane be as free as possible from impurities. In fact, intrinsic silicon is often desired. However, boron, because it is a fast diffuser in silicon, easily diffuses into the silicon membrane 7 leaving a residual impurity after the silicon boron layer is removed.
A further problem is encountered when the silicon membrane is used to form certain specialty electronic devices requiring intrinsic silicon of high carrier mobility. Since boron is a p-type dopant in silicon, the residual boron doped tail must be removed in a further processing step.
Still further, if the silicon membrane is crystalline and a high level of boron impurities is present, the crystal lattice is subjected to stresses that may cause the formation of dislocations in the material. Furthermore, for the silicon boron layer to function as an effective etch stop, the silicon must be doped with boron to at least 1.times.10.sup.20 atoms/cm.sup.3, with a resulting boron concentration of at least 0.2%. With this high concentration of boron, the likelihood of boron diffusing into the silicon membrane is high.
U.S. Pat. No. 5,013,681, which has the same Assignee as, and a common inventor with, the present invention, proposes using a silicon germanium etch stop layer in the formation of silicon on insulator substrates. However, such a silicon germanium etch stop layer has never been proposed in the processing field which concerns, for example, x-ray lithography, or in the specialty electronics field of balometers and the like.